The present invention relates to a bright triplet and, more particularly, to a behind-the-stop type triplet that has a wide field angle and is bright, so that it is well suited for use on photographic cameras.
So far, triplets have been used on many lens shutter cameras for the reasons that they can achieve relatively good optical performance with fewer lenses, and can be easily made compact.
Among these triplets, many proposals have been made of a behind-the-stop type wherein a stop is located behind a lens system, because its lens barrel is simple in construction and it is advantageous for drawing-out and exposure control.
However, since triplets have a limited degree of freedom because of being made up of fewer lenses, some inherent limitation is imposed on the achievement of wide-angle, bright and compact yet high-performance photographic lenses only by use of spherical lens elements. For this reason and to achieve such photographic lenses, many triplets having aspherical configurations have been proposed in the art. Of these, triplets including at least two aspherical lens elements, each made up of plastics, are set forth in JP-A 4-97111, JP-A 4-76510, JP-A 1-261613, JP-A 1-134412, JP-A 1-133016 and JP-A 62(1987)-183420. However, it is hard to say that all the triplets put forward by these patent specifications provide lenses having good-enough optical performance, because of being made up of plastic lenses. In particular, most of the lenses have an F-number of the order of 3.5 to 5; no bright lenses are obtained. Of these specifications, JP-A 62-183420 puts forward a triplet that is bright, as expressed by an F-number of about 2.8. However, this is still less than satisfactory in terms of correction for aberrations because there is chromatic aberration with no correction for coma. Thus, by use of plastic lenses it is impossible to achieve bright yet high-performance triplets.
Triplets comprising aspherical surfaces formed by using glass as vitreous material have been proposed in the art, as set forth in JP-A 59(1984)-34510, JP-A 59-160119, JP-A 59-160120 and JP-A 63(1988)-96620. The photographic lenses proposed by the publications mentioned above all use aspherical lenses formed by using glass as vitreous material, thereby maintaining good optical performance, while they are relatively bright and compact with a wide field angle.
Moreover, there are photographic lenses having aspherical configuration on both their lens surfaces so as to achieve better optical performance, as again set forth in JP-A 59-34510, JP-A 59-160119 and JP-A 59-160120.
However, these photographic lenses fail to provide a lens system which is brighter than expressed in terms of an F-number of about 3.5, as contemplated by the present invention.